There are numerous circumstances in which substances, in particular powders or granulates, are stored in silos or analogous enclosures so as to enable the divided substance to be extracted at will under controlled conditions with a determined quantity thereof being taken. As is known, the substance may be taken either at the outlet from a duct by means of a chute for the purpose of pouring it into a transport vehicle, or else the outlet consists in pipework enabling the divided substance to be conveyed directly to a particular point of use, e.g. some point along an industrial process. For example, this situation occurs in the chemical industry, in the food industry, or more generally in any industry that makes use of granular or powder substances.
In such an industrial situation, it will be understood that not only must the extraction apparatus enable the substance to be extracted in spite of it being possible that the substance might form a bridge, but also that it must enable said powder to be delivered at a substantially constant volume flow rate so as to enable the substance to be input into the industrial process.
French patent No. 75 38506 describes apparatus for fluidizing and extracting a divided substance, with the fluidizing of the substance stored in the silo opposing bridge formation. However that apparatus does not make it possible to control the volume flow rate at which the divided substance is delivered. In the above-described technique for avoiding bridging, the substance is fluidized, and that prevents flow rate being controlled since the apparent density of the divided substance is variable.
According to the above-mentioned French patent, the apparatus for extraction by fluidizing includes a shaft that extends inside the silo and that carries deformable flexible strips that are ballasted at their ends, and that are fixed to the shaft so as to occupy successive planes perpendicular to its axis, the lengths of the strips being no greater than the radial distances between the outsides of the hubs and the bottom wall of the silo.
Apart from the fact that the above system for extracting a divided substance by fluidizing it presents certain drawbacks, and in particular that of requiring the shaft to rotate at a very high speed (about 450 revolutions per minute (rpm)), that apparatus is also incapable of controlling the volume flow rate of the divided substance since it falls directly into drive means disposed at the base of the silo. It will thus be understood that the quantity of divided substance taken by the drive means varies quite substantially in time as a function of normal flow or of bridging phenomena that give rise to very different flow rates for the divided substance.
To remedy those drawbacks, an object of the present invention is to provide apparatus for extracting a substance in the divided state from a silo and for making it possible not only to extract the substance efficiently in spite of bridging phenomena, but also to ensure that the substance extracted in this way has a single and substantially constant flow rate.